An opposition proceeding is an administrative process available under the patent and trademark law of most jurisdictions which allows third parties to dispute the validity of a granted patent or trademark.
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In Europe, third parties may dispute the validity of a granted European patent by filing a post-grant opposition under the European Patent Convention (EPC).[1]
In Germany, third parties may dispute the validity of a pending German patent application under German patent law.
Under United States patent law, an opposition proceeding is called a reexamination.
In Japanese patent law, opposition procedure after an examiner's decision to grant a patent was abandoned in 2003; trial for invalidation serves as the alternative.
In the case of trademarks, third parties may use opposition proceedings to "oppose" the acceptance of a trademark application after it has been accepted and published for opposition purposes. If an opposition is defeated the trademark will proceed to registration. Some jurisdictions operate a "post-grant" opposition system, whereby opposition is not possible until after registration (eg. Japan).